Fibers and fabrics treated with curable phosphonates are rendered flame retardant. The phosphonate flame retardant agents are often fixed to the fabric substrate by heat-curing at temperatures in excess of 120.degree.C., however, heat-cure procedures frequently give rise to discoloration, typically a yellowing of the treated fabric. It is known that after treatment with aqueous solutions of sodium perborate or hydrogen peroxide at temperatures above 70.degree.C. will remove the discoloration.